The present invention relates to a process for the electrolytic production of cyanide in salt melts which contain cyanate and optionally carbonate and which are provided for nitrocarburizing steel.
The behavior in service of construction components is to a great extent determined by the behavior of the component surface. Nitriding or nitrocarburizing of steel has long been used on an industrial scale to increase not only resistance to wear and corrosion but also fatigue strength. Salt bath technology is of considerable significance in this connection. The principal components of these salt baths are the cyanates and carbonates of the alkali metals potassium and sodium.
Some patents, such as for example FR 9309706 or U.S. Pat. No. 5,518,605, are already known which relate to the performance of electrolysis in nitrating salt melts. However, these inventions solely relate to the improvement of the nitrocarburized surface of components. The components or feed materials themselves are here connected as anodes or cathodes. Patents concerning bath control of such melts, such as for example German 24 13 643 or German 25 29 412, by exploiting electrochemical potential measurement using the most varied electrodes are also known.
A process for the purposeful production of cyanide by electrolysis from the constituents of nitrocarburizing melts has, however, not hitherto been known. On the contrary, because of environmental and occupational health grounds, many variants of salt bath nitrocarburizing are directed towards avoiding cyanides in these melts.
The basis of the present invention, in contrast, is the recognition that a certain quantity of cyanide in the nitrocarburizing melts is highly advantageous with regard to achieving optimum serviceability (reduction of wear, corrosion protection) of the nitrided components.
It has long been standard practice to fuse nitrocarburizing melts without cyanide because the products may be transported and stored safely in this manner. Today, the fusing salts and replenishing salts conventionally used contain no cyanide at all. The cyanide advantageous for optimal nitrating is only formed over the course of a few days at operating temperature by gradual decomposition of the cyanate until an equilibrium is established in accordance with the reaction:
OCN.sup.- CN.sup.- +1/2 O.sub.2
The waiting times until equilibrium is established may be shortened by subsequently adding cyanide in the form of potassium and sodium cyanide or by fusing it when the molten bath is prepared or with the replenishing salt. The greatest problem in this case is the transport, storage and handling of the products containing cyanide. Occasional attempts have also been made to increase the speed with which a cyanate/cyanide equilibrium is established by adding iron chips or introducing scrap having an elevated surface area. However, none of the above constitutes a satisfactory solution to the problem of providing a nitrating bath fused without cyanide with the increased cyanide content necessary for optimum results. At present, no alternative possibility is known for producing a sufficient quantity of cyanide in such melts in another manner.
Therefore, it is an object of the present invention to produce cyanide in nitrocarburizing melts without using substances containing cyanide, in a short period of time and without impairing subsequent nitrocarburizing of components.